I. Field of the Invention
The present invention relates to wireless communications. More particularly, the present invention relates to a novel and improved apparatus and method of velocity estimation using AGC information.
II. Description of the Related Art
Wireless devices utilize radio waves to provide long distance communications without the physical constraints of a wire-based system. Information is provided to devices using radio waves transmitted over predetermined frequency bands. Allocation of available frequency spectrum is regulated to enable numerous users access to communications without undue interference.
A remote receiver tuned to a carrier frequency is required to receive and demodulate signals transmitted from a corresponding transmitter at the same carrier frequency. The remote receiver recovers the baseband signal from the modulated carrier. The baseband signal may be directly presented to a user or may be further processed prior to being presented to the user.
Portable wireless communication devices incorporating both a transmitter and receiver are used to provide two-way communications. Examples of portable wireless communication devices, commonly termed mobile units, are wireless telephones. Wireless phones may form a part of a wireless communication system such as those defined in Telecommunications Industry Association (TIA)/Electronics Industries Association (EIA) IS-95-B, MOBILE STATION-BASE STATION COMPATIBILITY STANDARD FOR DUAL-MODE SPREAD SPECTRUM SYSTEMS and American National Standards Institute (ANSI) J-STD-008, PERSONAL STATION-BASE STATION COMPATIBILITY REQUIREMENTS FOR 1.8 TO 2.0 GHZ CODE DIVISION MULTIPLE ACCESS (CDMA) PERSONAL COMMUNICATIONS SYSTEMS. Wireless phones used in the two aforementioned systems must conform, respectively, to the standards TIA/EIA IS-98-B, RECOMMENDED MINIMUM PERFORMANCE STANDARDS FOR DUAL-MODE SPREAD SPECTRUM CELLULAR MOBILE STATIONS and ANSI J-STD-018, RECOMMENDED MINIMUM PERFORMANCE REQUIREMENTS FOR 1.8 TO 2.0 GHZ CODE DIVISION MULTIPLE ACCESS (CDMA) PERSONAL STATIONS.
A radio receiver operates in a relatively hostile environment. A radio signal propagating from a transmitter to a corresponding receiver is subjected to scattering and reflections by objects and structures surrounding the transmitter and receiver. Structures, such a buildings, and surrounding terrain, such as walls and hillsides, contribute to the scattering and reflection of the transmitted signal. The scattering and reflection of the transmit signal results in multiple signal paths from the transmitter to the receiver. The objects that contribute to the multiple signal paths are centered about the receiver in a radius that is proportional to the receive signal wavelength. The contributors to the multiple signal paths change as the receiver moves.
The signal incident at the receiver antenna is the sum of all the multipath signals that are the result of the scattering and reflections of the signal from the transmitter to the receiver. The composite received signal can be modeled as having two components.
The first component is termed shadowing, slow fading, lognormal fading, or long-term fading. Slow fading results from the terrain contour between the transmitter and the receiver or as a result of the receiver passing through a tunnel, under a bridge, or behind a building. The received power measured at any particular location varies in time due to the effects of slow fading. The measured receive power due to the slow fading component is lognormally distributed.
The second signal component is termed fast fading, multipath fading, short-term fading, or Rayleigh fading. Fast fading results from the reflection and scattering of the transmitted signal by obstacles in the transmit path such as trees, buildings, vehicles, and other structures. Fast fading results in a fade of the entire receive bandwidth where signals arriving at the receiver combine destructively.
The signal incident at the receiver is composed of the fast fading signal superimposed on the slow fading signal. As a result, a moving radio receiver may experience tremendous variations in received signal strength. Additionally, a moving receiver experiences a frequency shift in the received signal. One contributor to a frequency shift is the doppler shift that causes a receive signal frequency offset proportional to the speed of the receiver relative to the transmitter.
A moving radio receiver, such as a mobile phone operating in the IS-95 or J-STD-008 communication systems, experiences signal fades and frequency doppler shifts as a routine part of its operating environment. A mobile receiver incorporates various techniques to compensate for the amplitude and frequency variations of the incoming signal.
However, many of the mobile receiver demodulation algorithms can be improved if the mobile receiver has knowledge of its velocity. Moreover, knowledge of the mobile receiver""s velocity can be used in conjunction with position determination algorithms. Additionally, the velocity of the mobile receiver can be provided as telemetry data to be transmitted to a remote site or as data available to the user. What is needed is the ability to determine the velocity of the mobile receiver using the signals that are incident on the receiver. The measurement of the mobile receiver""s velocity needs to be performed without burdening the communication system.
The present embodiments disclose a novel and improved velocity estimator having a signal processor that extracts one multipath from a received signal, a signal scaler and multiplier for scaling the received signal by a scale factor that is the inverse of any AGC gain, an instantaneous envelope calculator, a running RMS calculator, a level crossing counter to calculate the number of times the instantaneous envelope values cross a level crossing threshold, and a look up table that maps a level crossing number to a velocity estimate. The velocity estimator may also incorporate a FIFO for storing a predetermined number of instantaneous envelope values.
When the velocity estimator is implemented within a CDMA wireless communication device, the received signal is a composite CDMA signal and a single multipath can be obtained by despreading and accumulating a pilot signal over a predetermined number of chips.
The level crossing counter may incorporate hysteresis into the level crossing counting by incorporating a high level threshold and a low level threshold. The high level threshold may be generated as a first predetermined level, M, dB above one half the running RMS value. The low level threshold may be generated as a second predetermined value, N, dB below one half the running RMS value. The values M and N may be three in a particular embodiment.
The velocity estimator may use a normalizing multiplier to generate a normalized value by multiplying the instantaneous envelope value by a normalizing factor. In one embodiment, the normalizing factor may be 2/(running RMS value). When the normalizing multiplier is used, the level crossing counter uses predetermined level crossing thresholds.